The study had two stages. Stage 1 involved an opportunistic screening for hypertension conducted at the 2018 Beirut Marathon. During stage 1, participants whose measurements indicated hypertension were invited to take part in a RCT (randomized controlled trial). Stage 2 involved a RCT to evaluate the effectiveness of a behavioural intervention to increase the proportion of participants who sought further medical attention. The research team hypothesised that the intervention would increase the proportion of participants who sought further medical attention compared to a control group that did not receive the intervention.

This is a retrospective registration specifically focused on the RCT component of this study. The study was conducted by Nudge Lebanon in partnership with B4- development (formerly known as QBIU) and SmartScience. The RCT was not pre-registered because the collaborators were unaware that pre-registration would be necessary. The collaborators asked the University of Warwick to re-analyse and write the project up for publication after the study was complete. The University of Warwick is now seeking to retrospectively register the trial before submitting to an academic journal. The fact that this trial is retrospectively registered will be plainly stated in the manuscript and all analyses will be presented as exploratory.

This study had two stages. Stage 1 involved an opportunistic screening and Stage 2 involved in RCT. This registration focuses on the RCT but both stages are described here.

Stage 1. The opportunistic screening for hypertension was conducted around the 2018 Beirut Marathon. Screening tents were set up at the marathon village during the four days leading up to the marathon (7-10 November), where runners collected necessary materials for the run and could take up the screening. An additional two tents were set up during the marathon day were spectators could take part. Participants indicated their consent for the screening before taking part. During their participation they completed surveys about their demographics and health information (including whether they were hypertensive). After they completed the survey, nurses and trained team members measured their blood pressure using calibrated equipment. Those participants whose measurements indicated hypertension were invited to participate in the RCT.

Stage 2. For the RCT participants were randomly assigned to either a intervention or control group in a 1:1 fashion. Participants in the intervention group were sent a WhatsApp message 25 days after the marathon reminding them to seek help for their hypertensive condition. Participants in the control group were not sent this message. Then one month later the researchers called all these participants to ask wither they sought further medical attention. The main outcome was whether participants self-reported seeking further medical attention.

The research objectives were as follows:

Stage 1 screening: Evaluate if the opportunistic screening increased people's awareness of their hypertensive condition.

Stage 2 RCT: Evaluate if the intervention increased the proportion of participants who sought further medical attention.

Inclusion Criteria:

• objectively taken measurements at opportunistic screening indicated hypertension,
• provided informed consented to participate in the trial, and
• provided valid contact details for a smart phone with WhatsApp messenger

Exclusion Criteria:

• n/a

The research team will cannot make identifiable individual participant data available.

The de-identified participant data used in this papers analyses can be made available by contacting Kelly Ann Schmidtke (ORCID identifier: 0000-0001-5993-0358) and can be reused for further analyses.

The researchers who called participants to ask if they sought further medical attention was not made aware of the participants' conditions.